1. Field of the Invention
The present invention relates generally to a diesel engine and more specifically to a swirl or prechamber type diesel engine featuring a piston crown cavity which improves dispersion and mixing of the flame which is ejected from the swirl chamber.
2. Description of the Prior Art
FIG. 1 shows a so called "clover leaf" type flame dispersing cavity or recess arrangement 1 which is used in combination with swirl or pre-chamber type diesel engines such as disclosed in Japanese Utility Model pre publications Nos. 57-174725, 57-97123 and 58-8715.
With this type of engine, in order to lower the peak combustion temperature in the swirl chamber 2 and thus reduce the amount of NOx produced, a fraction of the injected fuel is released into the main combustion chamber prior to the spontaneous combustion of the main air-fuel charge in the swirl chamber.
However, in the absence of any strong turbulence or other mixing influence in the main combustion chamber, most of the fuel which is permitted to escape into the main combustion chamber tends to remain in or close to the trench portion 3 of the cavity. As the cavity 1 exerts only relatively weak dispersive influences on the flame which is ejected into the trench portion it tends to flow up and over the rounded nose portion 4 of the clover leaf type dispersion recess located opposite the trench 3. This induces the situation wherein the fuel in the trench portion tends to be entrained in a flame plume P having a shape essentially as indicated by the broken line.
Under these conditions the fuel which is released into the main combustion chamber tends to be carried by the initial flame ejection into a relatively cool section of the combustion chamber.
Accordingly, the relatively rich flame and the entrained fuel (particularly the latter) are not mixed effectively with the air available in the main combustion chamber and do not undergo sufficiently rapid and complete combustion to enable good engine output and low noxious emissions.
As a result of this phenomenon, during low load engine operation high HC exhaust emissions result while under high load smoke formation tends to be excessive.